1. Prior Art Technology
The present invention relates to a video signal frame search apparatus for enabling a specific frame (or field) of a video signal recorded on a magnetic tape of a VTR (Video Tape Recorder) to be specified by a user, and for the specified frame to be searched for and subsequently outputted to a video printer apparatus to be printed.
2. Prior Art Technology
A video printer apparatus can be utilized to print a single selected frame of a video signal that is produced by playback operation of a VTR. In the following specification and in the appended claims unless stated otherwise, the term "frame" is to be understood as signifying a field of a video signal in the case of a video signal which is not of field interlace format, and to signify a frame of a video signal in the case of a video signal which is of field interlace format with 2 fields per frame. In recent years, video printers have reached a high level of technology. Generally, the video printer apparatus includes a frame memory, an analog/digital (A/D) converter circuit for converting an input analog video signal (e.g. produced during playback operation of a VTR) to digital data form to be stored in the frame memory, and a digital/analog (D/A) converter circuit for converting video data values read out from the frame memory into analog signal form, to be supplied to a printer head. In the case of a printer having a color printing capability, the R, G and B information contained in an input video signal is also converted to digital data which are stored in the frame memory. Use of such a frame memory is generally essential, since the printing speed is very low, by comparison with the frame rate of a video signal. Various methods of differing complexity have been proposed for selecting a desired frame which is to be printed, from the stream of successive frames of the input video signal. One simple method is to provide a "memory" switch on the video printer apparatus, and to supply the playback video signal directly to the video printer apparatus and also to a monitor for displaying the video images. An example of a circuit arrangement for achieving this is shown in FIG. 5. In that case, the user observes the monitor display during video signal playback, and actuates the "memory" switch when it is judged that the position of the desired frame has been reached. The contents of the first frame to occur following the switch actuation (i.e. two successive fields, in the case of the usual field-interlace video signal) are then stored in the frame memory. Successive horizontal scanning lines of the data stored in the memory are then read out and supplied to the printer head, to print out the desired frame.
The above arrangement has the disadvantage that since successive frames appear on the display at a high frequency (e.g. 30 frames/second), it is almost impossible for the user to accurately select a specific frame by actuating the "memory" switch while observing the video display, so that the time of switch actuation will inevitably overshoot the timing of the desired frame by a substantial amount. Thus the user will in general have to make a number of successive trial attempts at printing out a desired frame, so that excessive time will be required in order to obtain satisfactory results.
Alternatively, it is possible to set the VTR in an operating mode in which successive frames are successively displayed during relatively long intervals on the monitor display (e.g. a step-advancement playback mode) and, when a point is reached at which a desired frame appears on the display, to set the VTR in the "still" playback mode of operation to produce a stationary picture on the monitor, and to then supply the playback video signal to the video printer apparatus. The desired frame can thereby be printed out. This method can be utilized with a video printer apparatus which does not include a frame memory.
However this method has the disadvantage that the video signal that is produced from a VTR during the "still" playback mode has a substantially lower signal/noise ratio than the playback signal that is obtained during normal-speed operation. The reasons for this are illustrated in FIG. 6. As shown, the video signal is recorded on successive tracks which are aligned at a fixed angle with respect to the direction of elongation of the magnetic tape, and are produced by successive scans across the tape by the magnetic heads of the rotating head cylinder during recording. An audio track extends along one side of the tape, i.e. along the longitudonal direction of the tape, while a control track having control pulses recorded thereon (described hereinafter) extends along the opposite side of the tape as shown. During normal-speed playback operation, the magnetic heads traverse the same paths as during recording, along these tracks. However during "still" playback operation, since movement of the magnetic tape is halted, the paths on the tape that are scanned by the rotating magnetic heads do not accurately coincide with the recorded tracks. As a result, the aforementioned reduction of the S/N ratio occurs during "still" mode playback, and hence the picture quality that is obtained by print-out using such a playback signal will be inferior to that which is obtainable by using a playback signal that is generated during normal-speed playback operation.
In the case of an industrial-use VTR which is provided with a VITC signal recording function, frame serial numbers (sometimes referred to as frame addresses) can be recorded on the magnetic tape for respective frames, with the frame serial data being inserted into the vertical blanking intervals of the recorded video signal. These frame serial numbers can be utilized by a frame search apparatus. However in order to print out a specific desired frame, in the prior art, it has been essential for the user to be aware of the exact frame serial number of that frame, whereas in general the user will only be capable of specifying a frame number which approximates to that of the desired frame. Thus even in the case of an industrial-use VTR having such a frame serial number function, it has not been possible for the user to easily and conveniently specify that a specific frame is to be searched for and outputted to a video printer.
Furthermore, a normal domestic-use VTR is not provided with such a frame serial number function, and a frame search apparatus which executes writing of frame serial numbers on the magnetic tape, for use with a domestic-use VTR, would be complex and expensive to produce.